Not merely slipping away: Forgetting requires biochemical action

From the editors and reporters of Scientific American , this blog delivers commentary, opinion and analysis on the latest developments in science and technology and their influence on society and policy. From reasoned arguments and cultural critiques to personal and skeptical takes on interesting science news, you'll find a wide range of scientifically relevant insights here. Follow on Twitter @sciam.

We’ve all suffered through the last-minute exam cram—that largely futile attempt to memorize as much as possible in the final minutes before a test. No matter how hard we try to remember it all, the information often disappears the minute we read the first question. Whereas forming memories is an active and often exhausting process, losing them seems to happen quite passively as time elapses and new information overloads our busy brains. But a new study published February 19 in the journal Cell shows that forgetting is a biochemically active process not unlike memory formation.

The authors of the study, Yi Zhong and his colleagues from Tsinghua University in Beijing and Cold Spring Harbor Laboratory in Long Island, N.Y., drew their conclusions based on studies of fruit flies. They created Drosophila melanogaster in which they could turn off or turn up a protein called Rac, which plays several roles in intracellular signaling. Then they had the tiny flies perform three memory-forming tests.

In the first, the flies learned to associate one of two odors with a negative consequence (a mild electric shock to a foot). In the second, the flies had to learn a new pair of odors and their consequences—a test called interference learning because it interferes with the memory of the first test. In the final test, called reversal learning, the consequences associated with the first pair of odors were reversed.

The researchers found that inhibiting the production of Rac slowed memory decay over time and suppressed forgetting: the flies performed better for longer on the first test but worse on the interference and reversal tests. Increased Rac activity had the opposite effect—it accelerated memory decay and promoted forgetting. The effects were independent of the formation of memories, because all flies acquired the memory of the first association equally.

Remembering has obvious advantages to our survival. But what about the benefits of forgetting? We might want to eliminate a memory if it’s no longer necessary, as in the interference learning test. We might also want to lose a memory that’s inconsistent with our current circumstances, as in the reversal learning test. The fly study shows that memory decay and forgetting are active processes at the molecular level—separate from those that make memories and, perhaps, no less important. That’s something to remember even before you cram for your next test.

7 Comments

From this description, these results also seem to be consistent with a fixed capacity memory model, analogous to computer memory, in which a memory occupies a physical resource of limited capacity. In an oversimplification, for example, fruit fly memory storage might be accomplished by occupation of a receptor by a Rac protein. To store a new memory, the receptors occupied by the first memory must be reused, requiring the first memory to be forgotten.

robert schmid – Thanks. Not trying to boor you with unnecessary details, but for completeness here’s my results regarding your suggestion. Long Term Potentiation and Hebbian Learning seem to relate to the biological affect of cellular memory storage. In terms of the computer technology I’m more conversant with, these biological theories are analogous to circuitry specifications for a semiconductor memory, for example. This is a level of detail that few computer users have any need for (nor I, in this case). Most users have no need to understand how their text or data is encoded or physically stored, only that it can be found in named files within directory structures. Logical memory management is to some extent independent of the physical storage medium: a relational database could be implemented on optical disks or semiconductor RAM, although each has specific operational considerations, such as volatility.

I was attempting to suggest that the study results might be produced simply because retention of the memories required to learn the last two tests utilized the physical storage medium as retention of the memories required to learn the first test. In other words, that only file 1 or both files 2 and 3 could be stored on the fruit fly’s 1 GB USB memory drive.

In this study, Test 1 required memorizing two odors, associating a stimulus with one. Test 2 required memorization of two new odors, associating a stimulus with one. Test 3 changed the association of the stimulus from one odor to another. If, for example, the fruit fly could memorize a maximum of two odors, Test 2 could not be learned without forgetting the odors memorized in Test 1. In this case, to the extent increased Rac `activity’ improved odor memory retention, it would implicitly inhibit learning new odors. More specifically and succinctly, I was attempting to point out that their study may not have controlled the possible affects of memory capacity on test results. However, this presumes that fruit flies are not capable of concurrently memorizing four odors: an unlikely circumstance. Never mind…

If memory is indeed analogous to PC memory, we need the ability to "defrag" I know in my own case that I remember bits & pieces of a memory for very long periods of time while having forgotten the main context of the memory. Very interesting indeed.

jpWxMan – My personal information management perspective is that short term memory is optimized for fast storage, whereas long term memory is optimized for contextual storage efficiency. This requires a migration process analogous to database reorganization/defrag, which likely occurs as some part of the dream process. For the most optimal storage, events could become a list of pointers to previously stored snippets. This could also produce the fragmented recall of incorrectly stored memory sequences. But I’m just guessing.